Suppose a scientist is doing an experiment to determine the effect of an all-organic diet on the occurrence of cancer in rats. Which of the following would be the best control group for this experiment?
A. Rats fed a standard rat diet (not organic)
B. Rats fed an all-organic diet
C. Pats fed an all-organic diet, but half the amount of normal
D. Rats that are not fed anything

Hmm... I'm not 100% sure but I'd say it was D.

sorry, is there a picture? i do not see any arrows.

wut? unius0123 wut?

they had the law tat factories that produces smoke cannot be built near residential buildings.

building of proper sewage system was done to ensure that the waste is carried away from the humans so that they do not get diseases from it.

also they built water systems to provide water in those areas which are drought prone.